High-efficiency gas turbine engine

ABSTRACT

A high efficiency/low nitric oxide emission gas turbine engine includes an interchanger for cooling a compressed air side stream which is used for cooling the turbine section. The heated fuel is then mixed with steam for injection into a combustor, thereby reducing nitric oxide emissions, while preventing condensation during mixing. Where water is used for injection instead of steam, both the fuel and water are preheated by interchange with the hot compressed gas stream. Preheating the fuel and/or water prevents steam condensate or water from entering the combustor, and does so using heat available within the gas turbine cycle thereby increasing overall turbine efficiency. In addition, utilizing a fuel/air interchanger reduces the size of the heat exchanger and the need for other auxiliary equipment, reducing costs while increasing turbine efficiency.

This is a continuation of application Ser. No. 389,630, filed Aug. 4,1989, now abandoned.

TECHNICAL FIELD

This invention relates to axial flow gas turbine engines and moreparticularly to land-based gas turbine engines requiring high efficiencywith reduced noxious emissions.

BACKGROUND OF THE INVENTION

Gas turbine engines include a compression section, a combustion sectionand a turbine section. An annular flow path for working medium gasesextends axially through these sections of the engine. Generally, airenters the compression section where it is compressed, then passes intothe combustion section, where the pressurized air is mixed with a fuel(gas or liquid) and burned. The hot pressurized gases which result arethen expanded through the turbine section to produce useful work, forexample, by driving a generator to produce electricity or by driving apropeller in a marine propulsion system.

The overall efficiency of a gas turbine is a function of compressor andturbine efficiencies, ambient air temperature, nozzle inlet temperatureand type of cycle used. Most gas turbine installations are of the opencycle type using atmospheric air as the working medium and burningrelatively clean fuels such as natural gas.

Simple cycle gas turbines are relatively inefficient, with almost alllosses occurring in the hot exhaust gases. When exhaust gases can beused in a boiler or for process heating, the combination of a turbinewith a heat recovery apparatus results in a high efficiency power plant.Another method that results in high efficiency is to integrate the gasturbine with other process requirements.

For maximum efficiency, the turbine section must operate at the highesttemperatures possible. However, high temperatures have a negative impacton turbine life. Therefore, to balance these two factors, cooling air isusually injected into the turbine, with the air flowing inside theturbine blades and vanes to cool them while they are in contact with thehot combustion gases. This air is obtained by taking a side-stream ofcompressed air and injecting it into the turbine section.

In land-based gas turbines, there has been a continuing trend towardsimproving thermal efficiency while reducing noxious emissions. One ideafor improving efficiency involves precooling the turbine cooling airprior to its entry into the turbine. Such cooling increases the densityof the air and increases the temperature differential, reducing theamount of cooling air needed to meet turbine cooling requirements. Thisreduces a loss to the cycle, by increasing the amount of compressed airwhich passes into the combustor, improving overall efficiency.Typically, this cooling air is obtained by passing the compressed airthrough a fan cooled heat exchanger for rejecting the heat ofcompression to the atmosphere.

To meet emission requirements for noxious gases such as nitric oxides(NOx) produced in the combustion cycle, water or steam is injected intothe combustor, quenching the hottest combustion zones. Preventing a widetemperature gradient in the combustor would also minimize nitric oxideformation.

In liquid fueled turbines which produce steam by heating water with thehot exhaust gas, separate steam injection into the combustor istypically used for nitric oxide control. Water may be used with liquidfuels, by mixing with the fuel prior to injection in the combustor.

With gaseous fuels such as natural gas, steam is directly injected intothe fuel gas, avoiding a separate steam injection manifold. Althoughsteam can be injected separately into the combustor, NOx control isimproved if the steam is premixed with the fuel, thereby avoiding hotspots due to insufficient steam/fuel interaction in the combustor.However, the gaseous fuel must be heated prior to mixing to avoidinjecting a slug of liquid, i.e. steam condensate, into the combustor,which would cause instabilities in the combustion process or highthermal stress in the combustion chamber.

Part of the fuel heating may be accomplished by directing cooling airexiting the gas turbine enclosure to interchange through a heatexchanger with the entering fuel thereby preheating the fuel up to about70° C. However, a relatively large heat exchanger is required as thecooling air is at atmospheric pressure and is relatively cool. Inaddition, further heating, up to about 120° C., must be accomplished bya second heat exchanger to prevent condensation. Therefore, low pressuresteam must additionally be used, a thermal energy loss, requiring aseparate heat exchanger with associated piping. Alternatively thisheating could be done with heat from the turbine exhaust gas, but thisreduces the heat available for producing steam.

Another alternative is to use highly superheated steam to heat the steampipes, thus allowing a degree of cooling without condensation. However,this similarly reduces efficiency and continues the risk of watercondensation after mixing with the fuel gas.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a gas turbine enginewhich achieves high efficiency by reducing thermal energy losses.

It is a further object of the present invention to provide a gas turbineengine which reduces noxious emissions.

It is a further object of the present invention to provide a gas turbineengine which requires fewer heat exchangers, reducing the size and costof auxiliary systems.

According to the present invention, a gas turbine engine havingcompression means, combustion means and turbine means further comprisesinterchange means through which a compressed air side stream, exitingfrom the compressor means, passes, reducing the temperature of thecompressed air side-stream, the interchange means increasing thetemperature of a fuel, passing through the interchange means, to atemperature sufficient to prevent condensation of steam when the fuel ismixed with steam, prior to injection into the combustor means.

Interchanging the compressed air side stream with the fuel gaseliminates the need for withdrawing steam from the cycle whileadditionally returning the heat generated in the compression cycle tothe combustion chamber, thereby increasing the overall thermalefficiency. This eliminates or reduces the need for a cooling system forinterchange with the atmosphere, reducing the overall equipmentrequirements. The pressurized air is of a temperature sufficient toprovide more than enough heat to preheat the fuel gas to the requiredtemperature. Since no steam heat exchanger is required for preheatingthe fuel gas, the associated cost of the piping and a steam heatexchanger is eliminated. In addition, two large heat exchangers plus afan system for cooling the side stream are eliminated, while the actualheat exchanger provided for preheating the fuel gas is small in size asthe fuel and the air are both under pressure and at high temperatures,increasing heat transfer efficiency.

The apparatus for preheating the fuel gas is adaptable to systems wherewater is used instead of steam for nitric oxide suppression. In such asituation, both the water and the fuel are heated by the compressed airstream in separate heat exchangers prior to injection. Of course, thewater alone could be heated, and this system is adaptable to gasturbines which utilize a liquid fuel, as the liquid fuel may besimilarly heated with the compressed air.

In another embodiment of the present invention, an inert liquid may beused as an intermediate heat exchange medium to avoid the risk of across-over leak during interchange, which could create a hazardousfuel/air mixture. Generally, the inert medium passes through two heatexchangers, a first heat exchanger where it is heated by the compressedair and then through a second heat exchanger where the heat istransferred to the fuel for preheating. While such a system may increaseequipment costs, it should be noted that the sizes of the heatexchangers will still be small compared to the previous designs as thestreams will be under pressure and heat transfer efficiency will beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a prior art gas turbine cycle utilizing a fan type cooling airheat exchanger.

FIG. 2 is a gas turbine cycle having an interchanger for preheating afuel with a compressed air stream prior to steam mixing.

FIG. 3 is another embodiment of the present invention using a liquidfuel and liquid water injection for reducing noxious emissions, bothpreheated with compressed air.

FIG. 4 is another embodiment of the present invention using closed cycleinterchange with an inert heat transfer fluid.

FIG. 5 is another embodiment of the present invention, adapted toaccommodate both gaseous and liquid fuels in one system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a typical gas turbine cycle is shown including acompressor 1, a combustor 2, a turbine 3, with the turbine cycle of thesingle shaft type for driving a load 4. A compressed air stream 5 iswithdrawn from the compressor, cooled in a heat exchanger 6 throughinterchange with the air and directed into passages in the turbine 3 forcooling the turbine blades and vanes. Heat exchanger 6 may be of the airto air, or air to water to atmospheric air type.

An exhaust stream 7 enters a waste heat boiler 8 to produce steam 9. Aportion of the steam 9 is withdrawn from the waste boiler for preheatingeither a liquid or gaseous fuel 10, in a heat exchanger 11, as well asfor mixing with the fuel in a mixer 12 for reducing nitric oxideproduction in the combustor 2. The steam and compressor heat lossesreduce the thermal efficiency of the turbine, and require an additionalcapital investment in equipment. While the steam exiting the heatexchanger 11 could be expanded in a steam turbine, the losses incurreddue to pressure drops and heat loss make this impractical.

Referring to FIG. 2, a gas turbine 20 having an interchanger forimproving the thermal efficiency is shown. A compressor 21 utilizes airas the working medium with air entering at an end 22 and exiting in acompressed and heated state at 23. Generally, the air enters at atemperature of about 59° F., at atmospheric pressure. The compressed airexits the compressor at a temperature of about 750° F. at a pressure ofabout 235 psi.

The compressor 21 supplies the compressed air to a combustor 24.Generally, the combustor may comprise one or more chambers where a fuel25 is ignited with the compressed air to form a hot combustion gas 26for driving a turbine 27. The turbine 27 then drives a load 28, andexpels a spent exhaust gas 29. The exhaust from the turbine enters aboiler 30 which heats water to produce steam 31 while reducing theexhaust gas temperature prior to discharge.

In order to reduce emissions, the temperatures within the combustor mustbe controlled to prevent hot spots which result in the production ofnitric oxides. This is accomplished by adding steam or water to the fuelprior to injection into the combustor. A side stream (steam) 32 is takenand used for mixing with the fuel for nitric oxide suppression. The sidestream 32 is added to the fuel 25 in a mixer 33.

Referring still to FIG. 2, the fuel 25 generally arrives at aboutambient temperatures from a source of supply such as a natural gas feedmain or may be supplied at somewhat elevated temperature after exiting abooster compressor. Typically, the gas may be supplied at from 60 to 400psi, at temperatures of from 59° to 300° F. Should steam be mixed withthe fuel for reducing nitric oxide emissions, it is possible that someof the steam would condense within the pipe and impinge on the cumbustorwall, causing combustion instability or high thermal stress. Therefore,a preheat interchanger 34 is included in the fuel supply to the mixer33. The fuel 25 is preheated by interchange with a compressed coolingair stream 35. Thus part of the heat of compression is transferred tothe fuel and returned to the turbine cycle when injected into thecombustor. If liquid fuel is used, the degree of fuel heating is limitedto a temperature below the fuel coking limit which is between about200°-300° F.

The cooled air stream 35 continues to the turbine inlet and is used tomaintain the turbine blade temperatures within the limits of thematerials of construction. Depending on the relative heat loads andinitial temperatures of the fuel and compressed air, it may be necessaryto have a second heat exchanger for rejecting heat to the atmosphericfrom the cooling air 35. This heat exchanger would be smaller and oflower cost than it would have been if the fuel heat exchanger were notused.

Referring to FIG. 3, a compressor, combustor and turbine are shown aspreviously described, however the gas turbine cycle utilizes a liquid orgaseous fuel and water rather than steam for nitric oxide suppression.Usually, the water is sprayed into the combustor with the fuel to avoidslugs of water disturbing combustion. In this embodiment, a firstinterchanger 36 is used to preheat a fuel stream 37, through heattransferred from a compressed air stream 38. The compressed air streamis split at 39, with a part of the stream entering a second interchanger40. The second interchanger 40 is used to preheat a water stream 41,prior to entering a mixer 42, where the heated water and fuel mix priorto injection. Of course, separate direct injection into the combustorcould also be used, avoiding the need for the mixer 42. The exitingcompressed air streams may be combined at 43, and continue on to aturbine 44. Alternatively, the fuel and water heat exchangers, 36 and40, could be in series rather than in parallel, relative to the stream38. In either case, interchange is utilized to preheat both the fuel andthe water prior to injection into the combustor. The temperature of thecompressed air is sufficient to generate steam instead of hot water inthe interchanger of this invention. Generally, economic and operatingconsiderations are used to choose between steam and hot water.

Through control of the cooling air stream 38, only the water or only thefuel may be preheated. For example, shutoff or control valves could beused to isolate either of the interchangers. This could produce the besteconomics depending on the amount of heat available in the cooling airside stream.

Referring to FIG. 4, an alternative embodiment of the present inventionis shown including an inert heat transfer fluid recirculated forexchanging heat between the compressed air and fuel gas streams. Such asystem may be desirable to prevent the possibility of a leak in theinterchanger causing the fuel and air to mix in an explosiveconcentration. A recirculation system 45 has a pump 46 and a reservoir47 for holding a heat transfer fluid 48 which may comprise a Dowthermtype material, or any suitable equivalent. The fluid 48 is pumped fromthe reservoir to a first interchanger 49 through which a hot compressedair stream 50 passes. The fluid 48 is thus heated and then passedthrough a second interchanger 51 through which a fuel 52 flows. Thus,heat gained from the compressed air stream is transferred from the fluid48 to the fuel 52. The fluid then returns to the reservoir 47 foranother cycle.

An addition to the system, where water must be preheated as well as thefuel, is shown in phantom in FIG. 4. Referring still to FIG. 4, a thirdinterchanger 53 is included in a water supply 54. The fluid 48 is splitat 55 with part of the heated fluid going to preheat the fuel in thesecond interchanger 51 and the remainder going to preheat the water inthe third interchanger 53. The fluid streams are combined at 56 andreturned to the reservoir 47.

Referring to FIG. 5, another embodiment of the present invention isshown which allows either a gaseous or liquid type of fuel to be used. Agaseous fuel 57 is supplied, through a valve 58, to a heat exchanger 59.An inert fluid 60 is circulated by a pump 61 in a closed loop throughthe exchanger 59 and a heat exchanger 62 through which a compressed airstream 63 passes. The compressed air stream is directed through either afirst valve 64 or a second valve 65, depending on the type of fuelsupplied. Where a liquid fuel 66 is used, a heat exchanger 67interchanges heat directly from the air stream 63 to the fuel 66. Steam68 similarly is directed by either a first valve 69 for direct injectioninto a combustor (not shown) or by a second valve 70 for mixing with thefuel 57 in a mixer 71. An auxiliary exchanger 72 is provided to furthercool the compressed air stream 63 prior to entering a turbine (notshown). This may be either an air or water heat exchanger. Thisarrangement is suitable for those turbines where fuel supplies and typesmay vary, allowing an operator to control the gas turbine interchangersfor optimum efficiency with minimized emissions.

While the preferred embodiments have been described in relation to a gasturbine engine for land based uses, it will be understood by thoseskilled in the art that the various other gas turbines could utilize thehigh efficiency/low emission gas turbine of the present invention.Consequently, it will be understood by those skilled in the art thatvarious changes or modifications could be made without varying from thepresent invention.

I claim:
 1. A high efficient gas turbine engine, having compressionmeans, combustion means, turbine means, and means for reducing nitrousoxide emissions from an exhaust gas stream exiting from the turbinemeans, the engine further comprising:interchange means in fluidcommunication with a hot compressed air stream exiting from thecompressor means, for cooling the compressed air stream as it passestherethrough; means for delivering the cooled compressed air stream tothe turbine means for cooling the turbine means, a fuel, in fluidcommunication with the interchange means, and separability passabletherethrough, the fuel heated by the hot compressed air stream prior toentering the combustion means and, pre-mixer means for reducing thenitrous oxide emissions, the heated fuel entering the pre-mixer meansand mixed with steam prior to entering the combustion means, theentering fuel being at a temperature which prevents condensation of thesteam during mixing.
 2. The gas turbine engine of claim 1 wherein theinterchange means comprise a heat exchanger, heat transferred betweenthe compressed air and the fuel in a non-contact manner.
 3. The gasturbine engine of claim 1 wherein the fuel is a gaseous fuel.
 4. The gasturbine engine of claim l wherein the fuel is a liquid fuel.
 5. The gasturbine engine of claim 1 wherein the interchange means is a first heatexchanger; and, further comprising a second heat exchanger, in fluidcommunication with the compressed air stream, and separably, in fluidcommunication with a water stream, for transferring heat from thecompressed air stream to the water stream.
 6. The gas turbine engine ofclaim 1 wherein the interchange means comprise a closed loop heattransfer system including an inert fluid; pump means for recirculatingthe inert fluid; a reservoir for containing a portion of the fluid;first heat exchanger means, through which the fluid is passed, thecompressed air stream separably passed through the first heat exchangermeans, wherein the fluid is heated thereby; second heat exchanger meansthrough which the fluid exiting from the first heat exchange means ispassed, the fuel separably passed through the second heat exchangermeans, wherein the fuel is heated to a desired temperature, the fluidthen returned to the reservoir for recycling.
 7. The gas turbine engineof claim 5 wherein the compressed air travels through the first andsecond heat exchangers in parallel.
 8. The gas turbine engine of claim 6further comprising a third heat exchange means, the fluid, exiting fromthe first heat exchange means which is in fluid communication with thethird heat exchange means, passing therethrough; and, water, separablypassed through the third heat exchange means, and being heated thereby.9. A method for operating a gas turbine engine having compression means,combustion means, and turbine means, the method comprising:providinginterchange means in fluid communication with a hot compressed airstream exiting from the compressor means; passing the compressed airstream through the interchange means for reducing the temperature of thecompressed air stream; delivering the cooled compressed air stream tothe turbine means for cooling the turbine means; passing a fuel,separably, through the interchange means for heating the fuel using thehot compressed air stream; providing pre-mixer means for reducingnitrous oxide emissions from the exhaust gas stream exiting from theturbine means; and, mixing the heated fuel in the pre-mixer means withsteam prior to entering the combustion means, the fuel being at atemperature which prevents condensation of the steam during mixing.